1. Field of the Invention
The present invention relates to electro-hydraulic braking EHB systems for motor vehicles.
2. Discussion of the Background
Electro-hydraulic braking systems for motor vehicles are known which comprise a brake pedal, a braking device connected to at least one vehicle wheel, which is capable of being brought into communication with an electronically controlled valve arrangement, in order to apply hydraulic fluid under pressure to the braking device, a hydraulic pump, and a hydraulic pressure reservoir fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking device via the electronically controlled valve arrangement, in order to apply hydraulic fluid under pressure, in proportion to the driver's braking demand as sensed at the brake pedal, to the braking device in so called `brake by wire` mode.
In the case of an electro-hydraulic braking system of this form, the braking energy required for braking the vehicle is provided in normal use by the electrically-actuated braking system ("brake-by-wire" mode). In order however, to brake the vehicle with the minimum of delay in the event of an unexpected failure of the electrically-actuated bring system, the braking system can also feature an ancillary hydraulic braking system for the immediate actuation of the brakes ("push-through" mode).
The area of application of an electro-hydraulic braking system of his form encompasses anti-locking brake system control, anti-slip control, electronic brake force distribution, and Car Dynamic Control, or "CDC".
In view of the fact that, with an electrically-actuated braking system, the driver's wishes are acquired by sensors at the brake pedal, and conducted to the electronic control system by means of electrical signals, such systems are described as electronic braking systems (EBS) or "brake-by-wire" systems.
To provide the redundant hydraulic emergency system, a direct connection can be established between the brake pedal and the brakes, in a conventional manner, by means of switch over valves and hydraulic lines, which is referred to as "push-through". This system requires a switching device by which, under normal operating conditions, the brake pressure which is produced in the electrical system, and, in the event of a defect in or the failure of the electrical system, the brake pressure produced in the hydraulic ancillary system, is transferred to the brakes.
An electro-hydraulic braking system capable of operating using the "push-through" technique is known from U.S. application Ser. No. 721,019.
The control valve arrangement is usually designed as a slide valve, with the result that leakages and lack of fluid tightness are inevitable. Consequently, a drop in the pressure of the hydraulic fluid contained in the hydraulic pressure reservoir cannot be avoided, in particular if the vehicle remains out of operation for extended periods. In addition to this, with the conventional system, the components associated with the hydraulic pressure reservoir (lines, seals, connections, etc.) are constantly subjected to sustained high, and cyclically varying pressure levels which prevail in the hydraulic fluid supply system. This can lead to premature material fatigue, leaks, and similar problems.
According to the known system of DE-A-4115356, there is provided an electrohydraulic braking system wherein a main pressure accumulator supplies the pressure for operating the brakes under the majority of operating conditions but whose maximum value is less than the maximum possible braking demand pressure which may be required to be met in use of the associated vehicle. Upon recognition of a braking demand which exceeds that which can be achieved using the main accumulator, a second, supplementary accumulator is arranged to be switched in. However, this system can supply at higher than normal pressures only the volume stored in the supplementary accumulator. Once the latter volume has been consumed, the pump will re-pressurise the brakes at the same time as it is re-pressurising the main and supplementary accumulators, ie. relatively slowly.
According to DE-A-4328304 there is provided a vehicle having an arrangement for braking the vehicle automatically when a collision with another vehicle running ahead is predicted. The system has an oil pump and a single pressure accumulator. Under normal braking, fluid is supplied to the brake lines via a pair of depressurizing solenoid valves. When a collision is predicted and the vehicle is running at a low speed, the depressurising valves are closed, a pair of pressurizing solenoid valves are opened, and the pump is energized so that a relatively low pressure is supplied to the brake lines from the pump via the open depressurizing valves. When a collision is predicted and tie vehicle is running at a high speed, the repressurizing valves are closed, the pump is energised and the depressurizing valves are repeatedly opened and closed so that a higher pressure is supplied step-wise to the brake lines from the accumulator via the depressurizing valves. Again, the maximum pressure that can be supplied is limited to the low pump pressure at low vehicle speeds and to the (higher) accumulator brake pressure at medium or high vehicle speeds.